Magnetic resonance tomography (MRT) is a modern method of examination with makes possible high-resolution imaging of slices of a person to be examined. Images of different types of tissue, especially also soft tissue, can be recorded with this method with high contrast. To expand the high-resolution image data with a functional imaging, MRT can be combined with Positron Emission Tomography (PET). Image data recorded with a combined MR-PET system and can thus deliver locally highly-resolved image information as well as functional information from the same region of the person under examination. In PET a person under examination has a weakly radioactive substance administered to them, the distribution of which in the organism is made visible by the decay of positrons which are emitted by the radionuclide. In such cases coincident decay events are detected and evaluated with annular detectors. On their passage through the material, such as the body of the person under examination for example, the photons arising during the decay can be absorbed, with the probability of absorption depending on the distance that the photons travel through the body of the person under examination. Accordingly a correction of these signals in relation to the attenuation by anatomical structures which are also located in the beam path is necessary in PET. For a combined MR-PET system such an absorption correction can be undertaken on the basis of recorded MR image data since this allows the position and contour of the person under examination to be reconstructed.
However the field of view (FoV) is limited in MR imaging, to 50 to 55 cm in the trans-axial direction for example. In particular a B0-homogeneity reducing at the edges of the field of view and a non-linearity of the magnetic field gradients in the outer areas of the field of view are responsible for the restriction of the field of view. This often leads to truncated or cut-off anatomical structures, such as truncated arms and shoulders for example, in the outer areas of the field of view. This problem is exacerbated in the examination of larger and overweight patients. In actual MR imaging the truncation of the structures does not represent any problem if the anatomical structures to be differentiated are located within the field of view. However for an absorption correction of PET data based on recorded MR image data, the truncation of the structures leads to significant errors. Parts of the body lying outside the field of view of the MR imaging can have a significant influence on the overall attenuation of the measured PET signal. Such problems arise in equal measure for radiation therapy which is planned on the basis of the MR image data.
With conventional methods this problem would typically be resolved by correcting of missing parts of the MR image data by means of an external body contour which has been extracted from uncorrected PET image data. This method is described in greater detail in Delso et al, “Impact of limited MR field-of-view in simultaneous PET/MR acquisition”, J. Nucl. Med. Meeting Abstracts, 2008; 49: 162P.”. Other approaches derive the missing image information of the body contour directly from PET raw data by means of different reconstruction methods. The missing information is provided in such cases exclusively on the basis of the PET raw data. Such methods are typically described in IEEE Trans. Med. Imag., vol. 18, pp. 393-03, 1999, “Simultaneous maximum a posteriori reconstruction of attenuation and activity distributions from emission sinograms” by J. Nuyts, et al, IEEE TRANSACTIONS ON MEDICAL IMAGING, VOL. 19, NO. 5, MAY 2000 451, “Reconstruction of Attenuation Map Using Discrete Consistency Conditions” by Andrei V. Bronnikov, and in IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 54, NO. 1, February 2007, “Activity and Attenuation Reconstruction of Positron Emission Tomography Using Emission Data Only Via Maximum Likelihood and Iterative Data Refinement”, Fabiana Crepaldi et al. However the disadvantage of these methods is that they require an initialization with a suitably selected start value and often converge in the direction of local and not global optima. These methods, since they are also based on PET raw data, must operate with incorrectly absorption-corrected data because of the truncations. Furthermore it is necessary for detectable amounts of the PET tracer to be located in the structures of the person under examination which are truncated in the MR image data, so that a signal is obtained from these structures. Extremities of the person under examination which are primarily affected by the truncation effects often only contain an insufficient amount of the PET tracer however and as a consequence are only shown badly contoured in the PET images. PET image data also as a rule have a significantly smaller local resolution than corresponding MR image data. Accordingly significant errors frequently occur in the correction of truncations based on PET data.
Furthermore there is an atlas and model-based method in which the PET absorption correction is undertaken on the basis of the model of the person under examination. Model or atlas-based methods can however not take account of the naturally occurring variance of body shapes and the actual position of the person under examination.
Other suggestions include the use of additional, external information sources, such as optical systems, ultrasound systems etc. for example. However the disadvantage of these methods is that additional measurement devices are needed and that in addition the person under examination in the MR-PET system is generally covered by local coils or other apparatus, so that detection of the body contour is rendered more difficult. Furthermore such measurement devices can have a disruptive effect on the MR-PET measurements.
Consequently it is desirable reliably to correct or to avoid truncations in MR image data with minimum effort.